1. Field of the Invention
This invention relates to a method of measuring Earth's crustal stress (to be referred to as "crustal stess", hereinafter) by hydraulically fracturing rock body through the use of a deep bore-hole and analyzing the manner in which the rock body is fractured. More particularly, the invention relates to such technical fields as exploitation of geothermal energy for energy resource development, earthquake prediction, underground stock of petroleum, and nuclear waste disposal.
2. Related Art Statement
Much research and development effort has been made these years in many countries to improve the method of measuring crustal stress, with the purpose of its application to exploitation of geothermal resources, earthquake prediction, disposal of nuclear waste, and underground stroage of petroleum. Since the above application has close relationship with industries and national welfare of any country, there is a widely recognized need for developing advanced techniques in the measurement. In view of such need, large-scale experiments on the measurement of crustal stress are currently undertaken, and studies are made on improvement of conventional methods.
Roughly speaking, there have been two groups of methods for measuring and evaluating crustal stress; namely, (A) stress-release method group, and (B) hydralic fracture method group.
FIG. 12 shows a typical stress-release method of the above group (A). A bore-hole 1 is drilled from the ground surface 4, and an over-coring hole 2 is bored by removing that cylindrical portion of the ground which surrounds the bore-hole 1. The over-coring hole 2 releases the stress, and the magnitude of deformation of the bore-hole 1 due to such stress release is measured by mounting a strain gauge 3 on the bottom surface 1a or the sidewall 1b of the bore-hole 1. The crustal stress is calculated from the released stress which is measured by the strain gauge 3.
Referring to FIG. 1, in a method of the above group (B), a portion of the bore-hole 1 is selected for the measurement and isolated by blocking the upper and lower ends thereof with packers 5. A hydraulic pressure is introduced to the isolated portion from a water supply system which includes a high-pressure pump 6, so as to effect hydraulic fracture of rock for producing a crack along the sidewall of the bore-hole 1. The crustal stress is determined based on the orientation of the crack thus produced and variation of the hydraulic pressure with elapse of time during the fracture in the isolated portion of the bore-hole.
On the above methods (A) and (B), the use of the methods (A) is limited to the close proximity of the ground surface, because, for deep bore-holes, the strain gauge is hard to mount in position and the output signal from the strain gauge is hard to detect. If a suitable tunnel is available for measuring personnel to reach a deep spot, then the methods (A) may be used as far as such personnel can reach. However, for the depth of several hundreds of meters or deeper, only the methods (B) are applicable.
In the hydraulic fracture methods (B), there are two kinds of cracks to be formed in the isolated section where hydraulic pressure is applied; namely, longitudinal cracks and traverse cracks. The longitudinal cracks are formed in parallel to the length direction of the bore-hole 1 (FIG. 2(a)), while the traverse cracks are formed so as to intersect with the bore-hole 1 (FIG. 2(b)).
The variation of the hydraulic pressure in the above isolated portion with time elapse is schematically shown in FIG. 3. In the figure, P.sub.b represents the pressure at which opening of a crack is suddenly increased in response to the delivery of high-pressure water to the isolated portion, P.sub.sb represents the pressure at which a crack that is once closed upon halting of high-pressure water supply is reopened after resuming high-pressure water supply, and P.sub.s represents the pressure when the water supply system is shut in. In the ensuing description, P.sub.b will be called the breakdown pressure, P.sub.sb will be called the crack reopening pressure, and P.sub.s will be called the shut-in pressure.
There are three types in the above methods (B) from the standpoint of crustal stress evaluation; namely, (i) basic type, (ii) longitudinal-crack-bypass type, and (iii) depth-proportional type.
(i) Basic type evaluation
It is assumed that one of major crustal stresses is vertical (vertical assumption). The crustal stress is evaluated by the following equations which relate to the longitudinal cracks. EQU P.sub.sb =-3.sigma..sub.h +.sigma..sub.H -P.sub.o ( 1) EQU .sigma..sub.h =-P.sub.s ( 2)
here, .sigma..sub.H, and .sigma..sub.h are principal stresses on a horizontal plane (.vertline..sigma..sub.H .vertline.&gt;.vertline..sigma..sub.h .vertline.), and P.sub.o is a pore pressure.
(ii) Longitudinal-crack-bypass evaluation
Referring to FIG. 4, the method of this type evaluation extends the longitudinal cracks beyond the packer 5, and the measurement is taken while causing leak of water from the above-mentioned isolated portion, which is pressurized, to a non-pressurized portion. In this case, the above equation (2) is replaced with the following equation. EQU .sigma..sub.h =-fP.sub.s ( 3)
here, f is a coefficient which is determined by laboratory experiments and numerical simulation, and its value is usually 0.6.
(iii) Depth-proportional type evaluation
The crustal stresses are assumed to be distributed in proportion to the depth (depth-proportionality assumption), and the proportionality coefficients are determined based on a large number of measured data on the pressures P.sub.s and P.sub.sb.
In reality, however, the crustal stress is affected by various subsurface conditions, or geological structural conditions, and both of the above vertical assumption of the (i) basic type evaluation and the depth-proportional assumption of the (iii) depth-proportional type evaluation are not necessarily appropriate. Especially, in zones where considerable underground crustal movement is present, such as the circum-pacific zone and Mediterranean coastal zone, and in geothermal zones where thermal stresses prevail, the above two assumptions are not realistic.
The above-referred (ii) longitudinal-crack-bypass type evaluation does not use any assumptions which predetermine certain strain conditions, but in order to fully determine crustal stresses at a given depth by this type evaluation, two bore-holes with different inclinations and hydraulic fracturing data at two portions of each bore-hole are necessary. Thus, this type evaluation is quite costly and requires a large amount of labor and time. In short, the longitudinal-crack-bypass type evaluation is unrealistic and is not practicable except cases where tunnel wall is available for combined use with shallow small-diameter bore-holes for desired measurement.